Summary Intracellular protozoal diseases, including visceral leishmaniasis (VL) and malaria, are predominant causes of morbidity and mortality of children throughout tropical regions. Therefore, there is a critical need to understand the roles of humoral responses during intracellular protozoan parasite infection and why effective long-lived humoral memory does not typically develop after infection with these pathogens. The interaction of nave B cells with T follicular helper cells (TFH) within the spleen, informed by the cytokine environment, is a crucial signaling exchange that establishes populations of effector and memory B cells. The specific subsets and effects of regulatory B cells on splenic structure and TFH function is unknown. Using our unique, naturally- occurring, canine model of VL that closely mimics human VL, we provide new evidence that pathological alterations in splenic follicular structure determine subsequent B and T cell dysfunction following Leishmania spp. infection. Using this model we identified mechanisms of Leishmania infection-induced IgDhi regulatory B cells; inhibiting macrophage reactive oxygen species production promoting T cell exhaustion via robust regulatory IL-10 production and expression of the checkpoint inhibitory receptor PD-L1, respectively. Our new data reveal novel mechanisms by which Leishmania parasites subvert host immunity and support our objective to; a) identify how specific B reg cell subsets inhibit production of long-lived humoral immunity and promote T cell exhaustion during progressive VL, b) understand why humoral immune memory is slow to develop in response to this disease, and c) identify mechanisms and pathways that can be therapeutically targeted to improve humoral immunity against intracellular protozoan parasites, including Leishmania. We will use the power of our natural canine VL model to establish the timing and cellular mechanisms of how regulatory B cells contribute to alter splenic structure and TFH function. We address these goals and test our hypothesis with experiments proposed in the following specific aims: 1) Determine the status of Leishmania-specific memory B cell subsets during controlled or progressive natural VL and if these cells prevent clinical VL, 2) Identify where within the spleen B regulatory cell interactions disrupt germinal cell function and T cell effector states in secondary lymphoid organs over time during natural VL, 3) Determine how PD-1 based T cell inhibitory mechanisms interact with regulatory B cells during infection to alter memory and overall immunity and clinical status. Completion of this proposed work will identify new translational opportunities to improve clinical outcomes of progressive VL through alteration of chronic inflammation, retention of functional intrafollicular germinal centers and recovery of memory B cells.